1. Field of the Invention
This invention is directed to a novel method of polymerizing poly(glycidyl nitrate) from high purity glycidyl nitrate synthesized from glycerol. This invention is also directed to methods for making explosive compounds, pyrotechnics, and solid propellants comprising poly(glycidyl nitrate) elastomer binders.
2. Description of the Related Art
Solid high energy compositions, such as propellants, explosives, gasifiers, or the like, generally comprise solid particulates, such as fuel particulates and/or oxidizer particulates, dispersed and immobilized throughout a binder matrix comprising an elastomeric binder.
In recent years, energetic polymers have been developed and evaluated as replacements of inert polymer binders in cast propellant systems, explosive compositions, and pyrotechnics. The substitution of an energetic polymer for an inert polymer in a typical pressable or extrudable explosive composition increases the detonation pressures and detonation velocities of the explosive. In this regard, much recent work has centered on attempts to produce acceptable energetic polyoxetanes and glycidyl azide polymer (GAP).
A problem with elastomeric binders formed from polyoxetanes is their relatively low oxygen balance. Also, it has been reported that polyoxetanes tend to have mechanical characteristics less than that which is desirable for some high energy applications, particularly for a rocket motor propellant.
Due to safety and toxicity concerns that arise during processing of glycidyl azide monomer, GAP is commonly synthesized by polymerizing epichlorohydrin (rather than glyciyl azide) to form poly(epichlorohydrin). The chlorine substituents are then displaced by reaction with sodium azide in dimethylsulfoxide. Thus, the desire to avoid the direct polymerization of glycidyl azide complicates the GAP synthesis route. Moreover, the resulting polymers have been reported as being characterized by low molecular weights and amorphous structures.
Poly(glycidyl nitrate) (PGN) has been known and recognized as a possible energetic polymer suitable for use in propellants, explosives, gas generants, pyrotechnics, and the like. PGN is most commonly synthesized in the industry by a three-step procedure characterized by a first step in which epichlorohydrin is nitrated, and a second step in which the nitrated epichlorohydrin is recyclizated with a base to form glycidyl nitrate. The glycidyl nitrate is then polymerized in a third step by cationic polymerization to form PGN. The selection of epichlorohydrin derives from the low cost of the reagent and the relatively high nitration yields obtained by the nitration of epichlorohydrin. Despite these relatively high nitration yields, in the subsequent recyclization step an appreciable amount of epichlorohydrin is regenerated with the glycidyl nitrate. The presence of epichiorohydrin during subsequent cationic polymerization is highly disadvantageous, since the epichlorohydrin, unless removed, will copolymerize with the glycidyl nitrate to decrease the nitro group concentration of the resulting copolymer. As a consequence of the incorporation of the epichlorohydrin into the copolymer, a substantially lower energetic characteristic is attained than had epichlorohydrin not participated in the polymerization reaction. In order to increase purity of the monomer to an acceptable level for polymerization of PGN, the epichlorohydrin is distilled from the glycidyl nitrate prior to polymerization. However, because glycidyl nitrate is a primary nitrate ester and thus highly explosive, distillation of glycidyl nitrate in unsafe and unduly expensive for large scale operations.
Another known, yet less utilized process for making PGN resides in treating glycidol with nitrogen pentoxide N2O5 in an inert solvent, then quenching the reaction mixture in aqueous solution, as discussed in U.S. Pat. No. 5,136,062 to Millar et al. However, as generally acknowledged in the art and taught by Millar et al., in this reaction sequence glycidol is commonly distilled prior to its nitration reaction. If the glycidol is not distilled, then glycidol oligomers will be present in the nitrated product, and will interfere with the polymerization reaction. Moreover, even when distillation is performed, there is a potential for thermally initiated autopolymerization of the undistilled glycidol unless the glycidol is distilled under vacuum at a relatively low temperature prior to nitration.
Thus, although it has been long recognized that PGN is an excellent energetic polymer candidate for such applications as propellants, explosives, and pyrotechnics, a need persists in the art for a low cost and non-hazardous synthesis route that produces glycidyl nitrate of adequate purity and sufficiently low moisture contamination to permit effective polymerization without distillation or other elevated temperature purification of the glycidyl nitrate or glycidyl nitrate precursor.
It is, therefore, an object of this invention to fulfill the long-felt need in the art outlined above by providing a method of synthesizing PGN from a glycidyl nitrate monomer precursor in which neither the glycidyl nitrate monomer precursor nor the glycidyl nitrate monomer must be subject to distillation or other elevated temperature purification prior to polymerizing the glycidyl nitrate monomer to PGN.
In accordance with the principles of this invention, the above and other objects are attained by a process comprising nitrating glycerol with at least one nitrating source in a solvent to form a nitrated glycerol solution comprising dinitroglycerin, treating the nitrated glycerol solution with at least one cyclizing agent to convert the dinitroglycerin into glycidyl nitrate, and polymerizing the glycidyl nitrate into poly(glycidyl nitrate).
One of the main advantages of this invention is the circumvention of the need for distillation or other vaporization techniques to remove nitroglycerin prior to polymerization of the glycidyl nitrate. Rather, the nitroglycerin can be carried along with the dinitroglycerin during polymerization, thus significantly reducing production and labor costs. In this manner, the glycidyl nitrate is not exposed to elevated temperatures sufficient to cause accidental explosion or deflagration of the nitrate ester. Still more preferably, the glycidyl nitrate is not heated above room temperature at any time prior to polymerization. Moreover, given the high energy performance of nitroglycerin, the nitroglycerin can optionally be retained with the PGN, i.e., not washed out, for subsequent processing and end use.
Other objects, aspects and advantages of the invention will be apparent to those skilled in the art upon reading the specification and appended claims which explain the principles of this invention.
It is generally known in the art that glycidyl nitrate can be hydrolyzed from dinitroglycerin, which in turn can be synthesized by the nitration of glycerol CH2(OH)CH(OH)CH2(OH) (also known and referred to herein as glycerin), as proposed by T. Davis, The Chemistry of Powder and Explosive (J. Wiley and Sons, Inc. 1943), the complete disclose of which is incorporated herein by reference. Preferably, the nitration of glycerol is performed with nitric acid as the nitrating agent. Another one of the advantages of this invention is that it is not necessary to use industrial grade pure nitric acid, i.e., 98-100 wt %; rather, 90 wt % nitric acid is suitable for this invention. It is also within the scope of this invention to use other nitrating agents, such as the following: mixed acids, such as sulfuric and nitric acids, or acetyl nitrate; nitronium ion salts, such as NO2BF4, NO2ClO4, and/or N2O5; and trifluoroacetic anhydride (TFAA) with ammonium nitrate, nitric acid, and/or Crivello reagents. The molar ratio of nitrating agent to glycerol is preferably in a range of from about 4:1 to about 5:1.
The nitration of glycerol produces the five different compounds: trinitroglycerin (or nitroglycerin), two isomers of mononitroglycerin, and another two isomers of dinitroglycerin: 
The cyclization of the dinitroglycerin into glycidyl nitrate is performed in the presence of an inorganic hydroxide, including alkali metal hydroxides such as sodium hydroxide, potassium hydroxide, and lithium hydroxide. Alkaline earth metals, and in particular calcium hydroxide, can also be used. Generally, two molar equivalents of cyclizing agent may be used: the first equivalent for neutralizing the nitric acid, and the second equivalent for cyclization. Although hydrolysis treatment of the nitrated glycerol solution in an alkaline environment converts the dinitroglycerin isomers into glycidyl nitrate, the two mononitroglycerin isomers and the trinitroglycerin do not cyclize into glycidyl nitrate. Rather, the mononitroglycerin isomers are hydrolyzed, becoming immiscible with the glycidyl nitrate and the nitroglycerin, and can be removed, separated, or recovered (e.g., by decanting) from the reaction solution at or below room temperature by decanting or the like.
To the surprise of the inventors, it was found that the polymerization of glycidyl nitrate into PGN can be conducted in the presence of the trinitroglycerin without distilling off the nitroglycerin or otherwise purifying the reaction solution of nitroglycerin via vaporization, since the nitroglycerin does not interfere with the polymerization of glycidyl nitrate.
In order to increase the yield of glycidyl nitrate from the glycerol, reaction conditions can be and preferably are selected to drive the synthesis of 1,3-dinitroglycerin. Preferably, the nitration reaction is conducted at a relatively low temperature not higher than room temperature, still preferably 0-25xc2x0 C., and still more preferably 10-20xc2x0 C. The reaction typically requires at least about 4 hours, more commonly a minimal of about 6 hours, due to the low temperature at which the nitration reaction proceeds. Practice of these conditions can result in a dinitroglycerin concentration of at least 50 mol %, with trinitroglycerin being present in concentrations of less than about 20 mol %, still more preferably less than about 10 mol %, to satisfy safety concerns.
The nitration step is preferably conducted in the presence of an acceptable heat sink medium, such as an inert halogenated hydrocarbon, such as methylene chloride, chloroform, and/or dichloroethane.
Polymerization of the glycidyl nitrate occurs as follows: 
wherein n is an integer essentially equal to the hydroxy functionality of a polyol co-initiator and x is an integer representing the repeating units, by forming a catalyst-initiator complex and reacting the complex with glycidyl nitrate and wherein the ratio of mols catalysts/mols hydroxyls in the initiator is about 0.5:1. The glycidyl nitrate monomer is added to the catalyst-initiator complex reaction mixture at a rate in which the monomer is used up (reacted) essentially as fast as it is added, and the reaction temperature is preferably maintained at a temperature within the range of from about 10xc2x0 C. to about 25xc2x0 C.
The polymerization reaction is a cationic polymerization process conducted using a polyol co-initiator and an acid catalyst. The acid catalyst may be selected from among protic and Lewis acids known in the art, including BF3, HBF4, PF5, BF3:THF, BF3 etherate, as well as other initiators such as triethoxonium salts, such as triethyloxonium hexafluorophosphate, triethoxonium hexafluoroantimonate, and triethoxonium tetrafluoroborate. The polyol co-initiator forms a preinitiator complex with the polyol, for example, butanediol is known to form a complex with boron trifluoride (BF3). The complete disclosure of U.S. regular patent application Ser. No. 08/233,219, which describes polymerization with triethoxonium salts, is hereby incorporated herein by reference.
The polyol initiator employed generally has sterically unhindered hydroxyl groups. The polyol is preferably a diol, although triols and tetrols can also be used. Suitable diols include, but are not limited to, ethylene glycol, propylene glycol, 1,3-propanediol and 1,4-butanediol. Suitable triols include, but are not limited to, glycerol, trimethylolpropane, and 1,2,4-butanetriol. A suitable tetrol is, but is not limited to, 2,2xe2x80x2-dihydroxylmethyl-1,3-propanediol. The molecular weight of the polyol is relatively low, preferably less than 500, more preferably below 300 and most preferably below about 150.
The acid catalyst is used at a low level relative to hydroxyl groups of the polyol. A much more controlled reaction occurs if the catalyst, such as a Lewis acid, is used at a molar ratio relative to hydroxyl groups of the polyol of less than 1:1, preferably from about 0.4:1 to about 0.8:1. If a protic acid is used as the catalyst, the ratio of hydrogen ions released by the acid catalyst to the hydroxyl groups of the alcohol is also less than 1:1, preferably 0.4:1 to about 0.8:1. By using a substantially lower level of acid catalyst, incorporation of a greater percentage of the polyol moieties internally within polymer molecules is achieved, cyclic oligomer formation is suppressed to a level of about 2 to 5% or less, and lower polydispersity (Mw/Mn) of less than 2 is achieved. As referred to herein, polydispersity is measured by GPC (gel permeation chromotography).
The cationic polymerization reaction may be carried out in a suitable organic solvent conducive to the cationic polymerization. If a solvent is employed, such suitable solvent is a non-protic, non-ether, inert solvent. Such solvents include, but are not limited to, halogenated hydrocarbon solvents, such as methylene chloride, chloroform, and 1,2-dichloroethane.
The polymerization reaction is conducted in a manner whereby the glycidyl nitrate is combined with the reaction mixture at a rate essentially equivalent to its rate of reaction, so that no effective net concentration of monomer is built up in the reaction mixture and the reaction temperature is maintained at a temperature within the range of from about 10xc2x0 C. to about 25xc2x0 C., preferably from about 11xc2x0 C. to about 17xc2x0 C., and still more preferably about 13xc2x0 C. to about 15xc2x0 C.
When the reaction of catalyst and initiator results in the formation of alkoxide groups in the catalyst-initiator complex, such as for example, the presence of alkoxide group compounds in the reaction mixture formed by the reaction of boron trifluoride etherate and 1,4-butanediol (or other monoalcohol, diol, or polyol), the resulting PGN products are low in functionality. Pre-reacting the polyol 1,4-butanediol and boron trifluoride etherate and then optionally removing diethylether under vacuum conditions produces a PGN product essentially free of alkoxide groups. If, however, the catalyst and initiator would not form products containing such alkoxide groups, such as when boron trifluoride gas is employed instead of boron trifluoride etherate, then prereaction of the catalyst and initiator and removal of potential alkoxide compounds is not necessary.
A more detailed discussion of technique for polymerizing glycidyl nitrate into poly(glycidyl nitrate) is disclosed in U.S. Pat. No. 5,120,827, the complete disclosure of which is incorporated herein by reference.
The PGN as obtained can be utilized in explosive compositions without the need for further purification or recrystallization steps. However, after quenching of the PGN with, for example, methanol, it is within the scope of this patent to purify the PGN by removing the residual nitroglycerin, although retention of the nitroglycerin may be desirable for some end applications.
PGN may be used in combination with conventional or novel propellant and solid explosive ingredients as the basis for formulating very high performance insensitive propellant and explosive compositions. Propellant and explosive compositions suitable for use with PGN are taught in U.S. Pat. No. 5,587,553 and U.S. Pat. No. 5,690,868, the complete disclosures of which are incorporated herein by reference.
Representative explosive materials that can be made with PGN, as the sole binder or one of a plurality of binders, include gun propellants, cast cure explosives, and extruable explosives.
Generally, gun propellants comprise about 15 wt % to about 40 wt % of binder and plasticizer (at a plasticizer to binder weight ratio of 0:1 to 3:1), 0-80 wt % filler, such as nitramine (e.g., RDX and/or HMX), and optionally 0.5 wt % to 5 wt % ballistic modifiers.
Cast cure explosives in which PGN may be used generally comprise as ingredients 5-20 wt % of PGN and optionally one or more binders, 0.5-3 wt % of one or more curatives, 0.25-2 wt % of one or more cure catalysts, and 20-80 wt % of one or more oxidizers, which may include ammonium perchlorate, ammonium nitrate, and nitramines such as HDX or RDX.
Typical formulations for extrudable explosives include 5-35 wt % of PGN and optionally one or more thermoplastic elastomers, 0-65 wt % of one or more oxidizers, 0-90 wt % of one or more explosive fillers such as nitramines, 0-40 wt % of metals, and 0-25 wt % of one or more plasticizers.
PGN may also be used in conjunction with the preparation of composite propellant formulations, including minimum smoke, reduced smoke, and smokey propellants.
Minimum smoke propellants generally include as ingredients the following: 4-30 wt % of binder, 0.5-3 wt % of one or more curatives, 0.25-2 wt % of one or more cure catalysts, 40-80 wt % oxidizers, 0-50 wt % of energetic solid fuels such as nitramines, and 0-30 wt % of one or more other plasticizers. Other additives, such as 0-5 wt % ballistic modifiers, may also be added.
Typical formulations for the reduced smoke propellants generally are similar to minimum smoke propellants. However, if ammonium perchlorate is selected as a component of the oxidizer and/or aluminum or aluminum oxide is selected as a component of the fuel, the ammonium perchlorate and aluminum are used in sufficiently low amounts to retain the desired reduced smoke properties. Generally, ammonium perchlorate is present in an amount of not more than 90 wt % and aluminum is present in an amount of not more than 3 wt % for reduced smoke propellants.
Typical formulations for the smokey propellants generally are similar to those of reduced smoke propellants, but do not contain undue restrictions on the smoke generating components, so that aluminum can be used in concentrations as high as about 22 wt % (as limited by combustion efficiency) and the ammonium perchlorate can be used in concentrations as high as about 80 wt % (as limited by theoretical performance).
Methods of preparing energetic formulations are generally known in the art, and are set forth in A. Davenas, Solid Rocket Propulsion Technology (1993) and R. Meyer, Explosives (4th ed. 1993).
The binders can be energetic, inert, or a combination (e.g., mixture, copolymer or terpolymer) thereof Representative inert polymeric binders that can be used singly or in combination with PGN include HTPB (hydroxy-terminated polybutadiene), PBAN (butadiene-acrylonitrile-acrylic acid terpolymer), PPG (polypropylene glycol), PEG (polyethylene glycol), polyesters, polyacrylates, polymethacrylates, CAB (cellulose acetate butyrate), or mixtures and copolymers thereof Representative energetic polymeric binders that can be used singly or in combination with PGN include poly-NMMO (poly(nitratomethyl-methyloxetane)), GAP (polyglycidyl azide), 9DT-NIDA (diethyleneglycol-triethyleneglycol-nitraminodiacetic acid terpolymer), poly-BAMO (poly(bisazidomethyloxetane)), poly-AMMO (poly(azidomethylmethyloxetane)), poly-NAMMO (poly(nitraminomethyl-methyloxetane)), copolyBAMO/NMMO, BAMO/AMMO, or mixtures and copolymers thereof, with PGN and GAP being preferred. The binder can optionally be halogenated, such as fluorinated ethylene propylene copolymer, chlorotrifluoroethylene and vinylidene fluoride copolymer, polyvinylidene fluoride, polydifluorochloroethylene, fluorinated polyethers, PVC, polytetrafluoroethylene, or mixtures thereof.
Representative oxidizers include AP (ammonium perchlorate), AN (ammonium nitrate), HAN (hydroxylammonium nitrate), ADN (ammonium dinitramide), HNF (hydrazinium nitroformate) or mixtures thereof, as well as the nitramines mentioned below.
Representative reactive metals include aluminum, magnesium, boron, titanium, zirconium, or mixtures thereof. The metals may be present as a powder, particles, and/or in other forms.
Energetic solid fuels (for propellants) or explosive fill (for explosives) that can be used in combination with PGN include the following: TEX (4,10-dinitro-2,6,8,12-tetraoxa-4,10-diazatetracyclo-[5.5.0.05,903,11]-dodecane), nitramines such as RDX (1,3,5-trinitro-1,3,5-triaza-cyclohexane), HMX (1 ,3,5,7-tetranitro-1,3,5,7-tetraazacycloocatane), and HNIW (also known as CL-20) (2,4,6,8,10,12-hexanitro-2,4,6,8,10,12-hexaazatetracyclo[5.5.0.05,903,11]dodecane); NTO (3-nitro-1,2,4-triazol-5-one); NQ (nitroguanidine); TATB (1,3,5-triamino-2,4,6-trinitobenzene); and DADNE (1,1-diamino-2,2-dinitro ethane).
PGN can also be used to prepare high solids ( greater than 90% explosive ingredient content) pressable or extrudable explosives. The pressable or extrudable explosives can also contain one or more inert and/or one or more energetic plasticizers. Representative inert plasticizers include DOA (dioctyladipate), IDP (isodecylperlargonate), DOP (dioctylphthalate), DOM (dioctylmaleate), DBP (dibutylphthalate), oleyl nitrile, or combinations thereof. Representative energetic plasticizers include BDNPF/BDNPA (bis(2,2-dinitropropyl)acetalibis(2,2-dinitropropyl)formal), TMETN (trimethylolethanetrinitrate), TEGDN (triethyleneglycoldinitrate), DEGDN (diethyleneglycol-dinitrate), NG (nitroglycerin), BTTN (butanetrioltrinitrate), alkyl NENA""s (nitratoethylnitramine), or mixtures thereof.
Exemplary curatives for some of the above-mentioned binders include isocyanates, and exemplary cure catalysts include Lewis acids, triphenylbismuth, alkyltin compounds, such as dibutyltindiluarate.
A list of representative ballistic modifiers include, by way of example, Lewis acids, iron oxide (Fe2O3), and lead and lead-containing compounds, such as lead salts and organometallic lead compounds.